Can't boot macbook air without power supply

Hello!

I don't used my macbook air about a half year. Yesterday I started it. The problem is, I can't start the macbook without power supply and when i've started it and disconnect it from the power supply the macbook turns off. When i start it again, im directly on my desktop and all windows are still opened. In the battery indicator is a X and when i click on it it says: "No battery available. Power supply: battery". How is that possible? I've reseted the SMC but it doesn't helped.

If you have an idea, how i can fix this problem please answer this thread!

Thank you!

gigy122

PS: Sorry, my English isn't so good because i'm from Switzerland.😉